1. Problem to Which Invention is Directed
Polymerization catalysts especially olefin polymerization catalysts (Ziegler) generally are of a wide particle size distribution which particles generally include a great many fines. It is known that the catalyst particles provide "templates" for the formation of polymer particles; the catalyst particles themselves must be the same shape as the desired final polymer powder shape, desirably with a relatively narrow particle size distribution. Conventional high molecular weight polymer powders made with usual catalysts generally include polymer fines. The polymer fines cause the polymer powder to be very difficult to handle. A technique that would increase the particle size of catalysts particles thereby substantially eliminating the catalyst particle fines would be highly desirable. In order to obtain larger powder particles, e.g., greater than about 75 microns, the catalyst particles themselves should possess a mean particle diameter of at least 10, preferably at least 20 and most preferably at least 35 microns.
2. Discussion of the Prior Art
The stereo-specific polymerization of .alpha.-olefins such as propylene is well known in the art. Polypropylene resin has become a well-established plastic in the world market. Polypropylene powder sales are also increasing rapidly and, at the present time, are increasing more rapidly than sales of polypropylene pellets. The popularity of the polypropylene powder derives, at least in part, from the rapidly expanding use of filled grades of polypropylene, particularly glass- or talc-filled grades. Additionally, energy savings accrued by the polypropylene manufacturer, by eliminating the extrusion step required in pelletizing are significant.
Most customers demand that polypropylene powder provide all of the resin quality normally obtainable in pelletized products and, in addition, that the powder possess the qualities of good flowability, low fines content, and no "clods", i.e., large aggregates of particles. A reasonably high bulk density is also desirable.
The particle size distribution of catalysts can be made narrower by controlled growth of catalyst particles. Polyolefin particle size distribution can be made narrower by agglomeration and compaction of polymer particles. Adhesive binders have been used in this latter technique; however, adhesives do not function well in catalyst compaction since they poison the active sites of the catalyst.
No prior art is known to the inventor that embodies the concept disclosed here for eliminating catalyst fines. Although a Harshaw publication entitled "Harshaw's Dustless Process" describes a technique of eliminating dust from inert powders, Harshaw has no contemplation for using the process to obtain catalyst particle agglomerates of increased particle size which can be employed as polymerization catalysts for the direct production of large size polymer particles.
Harshaw Chemical has also issued U.S. Pat. No. 3,838,092 and U.S. Pat. No. 3,838,064 which disclose and claim the generalized concept of using the polytetrafluoroethylene as an agglomeration technique. But, the art has no concept or contemplation of utilizing these techniques to overcome the rather special and unique problem of Ziegler catalyst particles as outlined in the parent application.